Ford F-150 Owner's Manual page 295

If you do not use the proper fuel filler cap, excessive vacuum in
the fuel tank may damage the fuel system or cause the fuel cap
to disengage in a collision, which may result in possible personal injury.
Choosing the right fuel
If your vehicle is a flexible fuel vehicle (FFV), use only UNLEADED
FUEL and FUEL ETHANOL (Ed75–Ed85)
Your vehicle was not designed to use fuel or fuel additives with metallic
compounds, including manganese-based additives. Studies indicate that
these additives can cause your vehicle's emission control system to
deteriorate more rapidly. In Canada, premium grade fuel generally
contains more metallic additives than regular grade fuel. We recommend
using regular grade fuel. In Canada, many fuels contain metallic
additives, but fuels free of such additives may be available; check with
your local fuel dealer.
Do not use fuel containing methanol. It can damage critical fuel system
components.
Repairs to correct the effects of using a fuel for which your vehicle was
not designed may not be covered by your warranty.
Cleaner air
Ford endorses the use of reformulated "cleaner-burning" gasolines to
improve air quality.
2006 F-150 (f12)
Owners Guide (post-2002-fmt)
USA (fus)
Maintenance and Specifications
295